We have achieved continuous-wave electrically-injected lasing operation at room-temperature in InP-based microdisks heterogeneously integrated on a SOI nanophotonic circuit. The microdisks were evanescently coupled with sub-micron SOI wire waveguides, resulting in up to 10 mu W waveguide-coupled unidirectional output power, with a measured slope efficiency up to 20 mu W/mA. A tunnel junction was used for efficient electrical injection with low optical absorption. The measured laser performance agrees well with calculations based on a standard laser model. This model suggests that considerable improvement in laser performance is possible.